Automatic merge from submit-queue (batch tested with PRs 41061, 40888, 40664, 41020, 41085)
Allow values to be wrapped prior to serialization in etcd
This adds a new value transformer to the etcd2 store that can transform
the value from etcd on read and write. This will allow the store to
implement encryption at rest or otherwise transform the value prior to
persistence.
* [x] etcd3 store
* [x] example of transformation
* [x] partial error handling
This is in support of #12742
bc60def8a8